Sound attenuating device



Oct. 15, 1935. R. B. BOURNE SOUND ATTENUATING DEVICE Original Filed April 50, 1934 2 Sheets-Sheet l laps.

w F a T V 1 0 mw x I k i 4 J M M w wm E ma M NW mm M Y Patented Oct. 15, 1935 UNITED STATES SOUND ATTE'NUATING DEVICE Roland B. Bourne, Hartford, Conn., assignor to The Maxim Silencer Company, Hartford, 'Conn., a corporation of Connecticut Original application April 30, 1934, Serial No.

723,085. Divided and this application Decembet-.14, 1934, Serial No. 757,552

9 Claims.

characteristics may be applied to the design of l silencers for use in connection with the exhausts or intakes of internal combustion engines, air compressors, blowers and the like. Further purposes and objects of the invention will be disclosed as the specification proceeds.

20 The acoustic sidebranches which are the main subject of this invention are designed and used on the basis that progressive change of phase as a function of distance occurs therein. For the purposes of the invention they will be referred to as 25- linear sidebranches. As an example of a sound Wave attenuating device employing linear sidebranches having progressive change of phase therein, reference is made to Bourne, United States Patent No. 1,910,672, May 23, 1933. 30 There are in general, two types of linear sidebranches, i. e., those open at both ends, called open linear sidebranches'and those closed at one end, called closed linear sidebranches. When acoustic sidebranches are used in connec- 35 tion with silencers for internal combustion engines and the like, the use of an open sidebranch may not be feasible since both exhaust gas and sound can leave through the open end. The open type sidebranch possesses certain acoustic 40 properties, however, that make it desirable from an acoustic point of view. It is well known that a cylindrical tube of uniform cross sectional area, open at both ends, is substantially one-half wave length long for its fundamental natural 45 period or frequency and also resonates to all substantially harmonic overtones thereof, both even and odd, whereas the closed tube of uniform cross sectional area is approximately one-quarter wave length long for its fundamental frequency 50 and responds to odd harmonics thereof only.

Where reactive sidebranches are used as attenuating means for sound waves associated with exhaust and intakes of internal combustion engines and the like, it is desirable to ofier attenuation not only to the fundamental frequency of the sound, but also all its overtones.

It is obvious that an acoustic sidebranch which will respond to and attenuate a full series of harmonic tones and be entirely closed except for the point at which it is coupled to the sound-bearing enclo- '5 sure or conduit is very desirable. Heretofore, it has been impossible to combine the advantages of a closed linear sidebranch with one showing a response to a full series of harmonic overtones, both even and odd.

I have discovered that a sidebranch in the form of -a complete cone, open to the main sound channel at its large end and of course closed off at its pointed end, will respond to and attenuate a complete series of harmonic overtones which are substantially multiples of the lowest frequency for which it resonates. In one aspect of the invention I employ such complete cones as closed linear sidebranches in an acoustic silencing device, these sidebranches responding to and attenuatmg in the main channel or enclosure to which they are acoustically coupled, bands of frequencies, more or less wide, corresponding to and dependent upon the fundamental irequency of the conical sidebranches and all substantially virltegral multiples thereof, both even and odd. In another aspect of the invention I employ as sidebranches, in an acoustic silencing device, structures having a cross-sectional area decreasing with the distance from their junction point with the main channel, 'sufiiciently slender to give substantial attenuation to at least one consecutive overtone of their fundamental frequency, and having an opening into the main channel large enough to make the sidebr'anch operate mainly as a linear acoustic element.

Referring to the drawings,

Fig. 1 shows a truncated cone with open base, acoustically coupled at its base to a main conducting channel;

Fig. 2 shows a complete circular cone, continued to the apex, acoustically coupled at its base to a main conducting channel; 7

Fig. 3 shows in diagrammatic form a sound attenuating device embodying a single complete cone open at itsbase;

Fig. 4 is a section on line 44 of Fig. 3;

Fig. 5 shows in diagrammatic form a silencing device embodying side branches of special form acting somewhat as truncated cones;

Fig. 6 shows a simple sound attenuating device embodying a completely conical sidebranch and a second sidebranch having somewhat the properties of a truncated cone;

Fig. 7 shows a compact silencer suitable for intakes and the like;

Fig. 8 shows a somewhat similar device; and

Figs. 9 and 10 show the theoretical operating characteristics of the devices illustrated in Figs. 2 and 6 respectively.

This application is a division of my prior application Serial No. 723,085, filed April 30, 1934.

In order to understand the operation of the devices of the invention it is useful to make use'oi impedance relations, since the operation of sidebranches in general can be completely expressed in terms of the acoustic impedance looking into" the branch.

For a truncated cone such as is shown in Fig. 1, the impedance per unit area at the base, or point of coupling to the main channel 40, in terms of the dimensions of the cone 4| having a partition or header 42 a distance 11 from the apex and a slant length of :m-a:1 may be mathematically shown to be conducting channel 44. In this case, :i=0, whence, substituting for 3:: its equivalent Le, Equation (1) reduces to 1 20:1- PQ'TP 0L 1 6 ext. (2)

tan

In the above equation the symbols have the following significances, which hold true also for the further equations below with the changes and additions there noted:

Z;=acoustic impedance looking into the sidebrranch.

f=frequency of sound wave. 0:211. C=velocity of sound in the medium.

Po=mean pressure of the sound transmitting medium.

=ratio of the specific heat of the medium at constant pressure to that at constant volume or, for waves of large amplitude, a function of this ratio.

, =density of the medium.

m1=slant distance from the apex of the cone to its closed end (see Fig. 1). 7

=s1ant distance from the ,apex of the cone to its Junction with the main lconducting channel, allowing for any necessary end correction.

It may be noted, in order to permit comparison with formulas for other types of silencing units given by diflerent authors, that In Fig. 1 the conical sidebranch is connected to the main channel at its larger. end, and diminishes in cross section area in a direction away from the main channel. In the case of a true cone this change in cross sectional area is a simple function of the distance, while for certain sidebranches to be described below, the area depends upon the distance from the channel in a somewhatmore complex manner. It should be noted particularly that the sidebranch decreases in area in a direction away from the main channel, as the performance is entirely diflerent if the cone is turned with its small end toward the channel. As will now be shown Equation (1) may be used as the .bas'is for determining many of the acoustical characteristics of either truncated or complete cones connected to the channel at their large ends.

In Fig. 2, the truncated cone of Fig. l is continued to the apex. In this figure, the cone 43 of length In is acoustically coupled to the main whence 2 35 a result identical to that applying to a cylindrical tube open at both ends commonly called an open tube.

We now have an acoustical resonating device, completely closed except for an opening 40 whereby it may be acoustically coupled to a main conducting channel and capable of being used to attenuate certain groups of sound frequencies comprising a fundamental group and all multiples thereof, both even and odd. The use of 45 the term group rather than frequency is Justifled by the fact that resonating devices of the acoustic type exert their influence not only upon the theoretical single frequencies of resonance but also upon neighboring frequencies. The exact nature of the behavior of this type of resonating device undervarious conditions will be disclosed as the specification proceeds, and the advantages of certain specific arrangements will be shown.

In order toshow the attenuating effect of a as complete cone acoustically coupled to a main conducting channel upon sound waves therein, it is proper to firstinvestigate the performance of a single such sidebranch, coupled to a relatively long main conducting channel. The attenuation or in decibels may be shown to be where S: is the area of the base of the cone.

S1 is the area of the main conducting In order to obtain a larger area for the base of the cone then exists in the main conducting channel, the cone may be disposed within a casing after the manner shown in Fig. 3. In this embodiment, the cone 5 is supported within the casing g,-

48 in any convenient manner, making due allowance for the passage of gas between the base'of the cone and the casing and between the end of the cone and the header 41. The specific supporting construction is not shown in detail as it is not necessary for a complete understanding of the invention. It is seen that the open base of the cone is located directly opposite to and closely adjacent the opening in the header 41 leading into the channel 48 which forms part of the main conducting channel through the device. In this particular embodiment of the invention, the space between the outside of the cone and the inside of the shell or casing is utilized to the purpose of attenuating sounds of relatively high frequency, such as bang and hiss noises usually associated with internal combustion engine exhausts. I have shown a simple baille plate system 49 to represent such a high -frequency attenuating means. Many arrangements of passageways, partitions, etc. may be used to advantage in this regard. The apex of the cone may be supported either by brackets or by a partition 59 extending from the casing to the cone and having therein holes for the passage of the gas therethrough. It is theoretically preferable, in this and other embodiments, that the sidebranch retain its conical shape up to the point of coupling to the main channel. In some cases, however, it maybe desirable for constructional reasons to depart from the normal conical shape adjacent the zone of coupling. The departure from the theoretical response occasioned by this slight change in shape will in most cases be; of no practical consequence.

Referring again to Fig. 9 is seen that maximum attenuation occurs at values of c --1r, 21r, 311-, 41, etc.

which is the same as would be obtained for an open cylindrical tube of the same length. Such a tube would have an attenuation peak at zero frequency. It is to be noted that the conical sidebranch does not offer attenuation to zero frequency, the attenuation decreasing continuously from a high value at to zero at as shown. The points of zero attenuation are not midway between the points of maximum attenuation, as they are in the case of'the open cylindrical sidebranch, but are displaced slightly therefrom, the displacement becoming less as the frequency increases. It can be shown, by a consideration of Equation (4) that the condition for zero attenuation is given by the relation.

The first few roots of this equation are known to be I 0, 1.43031r, 2.4591r, 3.47 11r be notedv at this time that the area under the curve is larger as the ratio 82/81 is made greater; and, since this area represents a measure'of the usefulness, in attenuating sounds of many frequencies, of the device for which the curve shows operating characteristics, it is frequently desirable to make S2 as large compared with S1 as is commercially feasible with proper regard for the limitations imposed by acoustic theory.

Fig. 5 shows an extremely simple embodiment of the invention comprising a casing 96, a main conducting channel 91 centrally disposed within said casing and of such length that openings are left whereby the two sidebranches 98, 99 are acoustically coupled to the channel 91 as shown. Said sidebranches 98, 99 are formed by a conical member I00 extending from one end of the conduit 91 to the inside of the casing 96 at a point opposite the other end of the conduit 91 and may be described as being of conlco-annular form. These sidebranches, it has been found by experience, conspire to offer maximum attenuation in the main conducting channel to sound frequencies substantially the same as those frequencies which would be highly attenuated by truncated true cones, closed at the small end,

open at the base, and having a ratio between the area of the base and the area of the header forming the truncated cone, of 9:1. The impedance of such a truncated cone may be written The resonating frequencies for such a sidebranch are given by wL 00L 2 tan The roots of this equation are found to be approximately .731r, 1.621r, 2.581r, 3.571,

For high frequencies, it is seen that these values approach the value where n is any integer. Where two such sidebranches'are separated a distance L along a main conducting channel, as in Fig. 5, the device forms a wave filter, the transmission and attenuation characteristics of which are given by A consideration of Fig. 5 further discloses that the larger the ratio 32/51, the more nearly the operation of the sidebranch 99 approachesthat of a true complete cone, while the operation of the sidebranch 98 approaches that-of a cylinder.

As has been shown previously, the combination of a cone and cylinder to form acoustic sidebranches results in useful frequency-attenuation characteristics.

Fig. 6 shows a simple constructed sound wave attenuation device having unique acoustic characteristics. It comprises a cylindrical casing I20 and end headers I2I fitted with inlet and outlet openings I22. Suitably mounted within said casing is a cylindrical member I23 open at one end and having a diameter D, cross sectional area 8:. The member I23 isclosed at the other end by a header I24 having a circular opening therein of diameter d and cross sectional area S3. Afiixed to the header I24 is a cone I25 extending coaxi ally within the cylindrical member I23 and oil substantially the same length,- as shown. It will be seen .that the main conducting channel I26 has coupled to it the conical sidebranch I21 and the conico-annular sidebranch I28. One preferred design embraces a certain relation between S: and 8:. By making S3=3/4S2 the conico-annular sidebranch becomes approximately equivalent to a truncated cone wherein, referring to Fig. 1,

and the attenuating effect of a sidebranch operating in accordance with Equation (10), on sound waves in the main conducting channel may be A plot of Equation (11) is shown in Fig. 10, curve M. Curve N, due to the conical sidebranch I 21,

is similar to that of Fig. 9. The combined eiiect' is shown at P. A feature oi. the embodiment of the invention shown in Fig. 6 is the short length, made possible by the telescoping of the cone within the cylinder, as shown.

Fig. 7 shows a compact silencer suitable for intakes and the like where but one pipe connection is necessary. It comprises a casing I30 having an imperiorate end header I3I and an and header I22 having an opening I33 therein which is large compared to the diameter of a centrally disposed conduit I34 which extends therethrough into the interior of the device, and provides means for attaching the silencer to the apparatus with which it is used- Wiihin the casing I" is a suitably mounted cylindrical member I" having openings I36 therein, headers I31 extending from each end of the member 1" to the centrally disposed conduit I34. A conical member I38 extends from one end of the interiorly disposed conduit I34 to the remote end of thecylindrical member I35 substantially as shown. The spacing of the various elements is such as to form themain conducting channel I partly as a centrally disposed tubular conduit and partly as an annular conduit. It is seen'that this construction aiiords two sets of acoustical sidebranches each coupled to the main conducting channel at distant points along its length and that each side branch system comprises two dissimilar conico-annular chambers in parallel. By a suitable use of equations already given and by the proper choice of the dimensions an, an, and an, it is possible to'design this device for practically any acoustic performance casing I4I to the conical member I42 and an annular header I43 extending from the conical member I42 to the centrally disposed conduit I44 together with the casing I45 and suitable openings I 46 conspire to form the four acoustical sidebranches I41, I48, I49 and I50 each of which may have different acoustic ,characteristics. Proper proportioning of the various elements of the device results in a relatively complete attenuation spectrum. In both Figs. '7 and 8 the path of the gases through the device is shown by arrows.

I claim:

1. A sound wave attenuating device comprising a closed acoustic sidebranch having a cross sectional area decreasing from one end of the sidebranch to the other, and acoustically coupled to a main conducting channel at a point intermediate the length of the sidebranch.

.2. A sound attenuating device comprising a main sound conducting channel having coupled thereto, at separated points along the length thereof, closed acoustic sidebranches in parallel, one of said paralleled sidebranches increasing in cross sectional area and another of said sidebranches decreasing in cross sectional area.

3. An acoustic sidebranch wherein change of phase or wave motion takes place, the cross sectional area of said side branch increasing continuously from one end thereof to the other, said sidebranch being acoustically coupled, at a point intermediate the length thereof, to an enclosure wherein sound waves may travel.

4. A sound attenuating device comprising a cylindrical casing, closed at one end and having inlet and outlet openings both at the other end thereof, a nested shell assembly interiorly disposed within said casing whereby is formed an outer annular main sound conducting channel and an inner sound conducting channel connected in series and extending from said inlet opening to said outlet opening in said casing, and means associated with both of said channels to selectively attenuate predetermined sound frequencies occurring in said channels.

5. A sound wave attenuating device in accordance with claim 4 wherein are acoustically coupled to said channels, pairs of unlike closed acoustic sidebranches in parallel.

6. A sound wave attenuating device in accordance with claim 4, said sound attenuating means comprising a linear acoustic sidebranch oi continuously changing cross sectional area acoustically coupled to said channels at a point intermediate the length of said sidebranch.

'7. A sound wave attenuating device in accordance with claim 4 having the parts so proportioned that the sound frequencies attenuated in said outer annular channel are intermediate the sound frequencies attenuated in said inner sound conducting channel.

8. A sound attenuating device in accordance with claim 4 wherein one or more closed acoustic sidebranches are acoustically coupled to each of said sound conducting channels and wherein parts are so proportioned that the lowest sound frequency attenuated by one of said side-branches does not correspond to the lowest sound frequency attenuated by another of said sidebranches.

9. A sound wave attenuating device comprising three nested cylindrical shells the innermost ing with the space formed between said intermediate shell and said conical partition.

I ROLAND B. BOURNE.

Certificate of Correction Patent No. 2,017,746. October 15, 1935. ROLAND B. BOURNE It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line 17, for U =0 read Z =O; and line 72, for then read than; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 19th day of November, A. D. 1935.

LESLIE FRAZER, [SEAL] Acting Commissioner of Patents.

Certificate of Correction Patent N 0. 2,017,746. October'l5, 1935.

' ROLAND B. BOURNE It is hereby certified that errors appear in the printed specification of the above numbered patent re%uiring correction as follows: Page 2, second column, line 17, for U =0 read Z and line 72, for then read than; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 19th day of November, A. D. 1935.

, LESLIE FRAZER, [SEAL] Acting Commissioner of Patents. 

